This invention relates to an apparatus and a process for dehydrating gases wherein the dehumidifier utilized is comprised of membranes having dehydrating capability. In addition, the invention relates to a process and an apparatus for improving gas dehydration systems which employ membrane dryers placed in intermittent service.
In the use of many gases, removal of water is desired. For instance, removal of humidity from air for a variety of reasons is frequently required. In the manufacture of pharmaceuticals, it is desirable to maintain the pharmaceutical product in a dry atmosphere during packaging as well as providing a dry atmosphere in the package. Other dehumidifying uses of gases include dehumidification of buildings for maintaining comfortable working and living areas during summer months, to providing dry air for communication systems, uses in the textile industry, in certain chemical processes, in the petroleum industry and in many other industrial fields.
Presently there are several methods for removal of water vapor from a gas. One method involves bringing gas into contact with a hygroscopic agent such as silica gel, a molecular sieve, quick lime, calcium chloride, phosphorous pentoxide, lithium chloride, or concentrated sulfuric acid or the like to remove moisture contained in a gas. When utilizing one of the above mentioned hygroscopic agents, it is always necessary to either dispose of or regenerate the used hygroscopic agent. Such disposal or regeneration does not permit continuous operation of the system utilizinq the gas without an alternate back-up for use during regeneration of the hygroscopic agent.
Another method for removal of moisture from a gas involves condensing moisture contained in the gas by compressing and/or cooling the gas to remove the moisture. Although this method permits utilization in a continuous operation, it is disadvantageous as the sole dehumidification method because it requires a large quantity of energy, and dehydration to a level below the freeze point is very difficult and hence costly.
In one of the more recently developed methods, water vapor is removed from a gas by using membranes having selective permeability to water vapor. An example of such a product is the PRISM.RTM. CACTUS.RTM. dryer manufactured by Permea, Inc., St. Louis, Mo. U.S. Pat. No. 4,783,201, Arthur W. Rice et al, hereby incorporated by reference, provides a suitable membrane separator and a method for utilization of membranes to effectively dehumidify a gas.
The above methods discuss systems or methods not suitable for continuous operation and one method suitable for continuous operation. It has been found that certain "continuous" operations are nevertheless interrupted because only intermittent requirements for a continuous dry gas are present.
When using a membrane process for removal of water vapor there are basically two processes; one in which a homogenous membrane is used, and one in which a porous membrane is used. For most membrane systems, a gas portion which also permeates the membrane or an alternate dry gas is utilized to sweep away the permeated water vapor. Generally, the sweep flow is countercurrent to the flow of the gas which is to be dehumidified. As a result, a large concentration gradient of moisture is present along the flow path. However, this use of a countercurrent sweep flow permits continuous high drying capability in a single compact membrane device. (See U.S. Pat. No. 4,783,201, Arthur W. Rice et al)
These membrane devices require a pressure gradient across the membrane. The pressure gradient allows for two methods of transport of a gas across the membrane. For the homogenous membrane, the gas must be adsorbed on one side, diffused through to the opposite side and then desorbed. For the porous membrane, the size of the pores allows flows at different rates for different sized molecules and those having different mean free paths. Some membrane gas separator devices, such as the above mentioned CACTUS.RTM. dryers, operate using both methods simply because a permeable surface is used which also has controlled porosity.
For cyclic pressure systems, such as in air compressor operations where an air compressor starts and stops, the performance of a membrane dehumidifier is subject to certain complexities. When the compressor starts at initial low pressure, very little separation of moisture occurs thereby allowing high humidity product to flow to the dry product outlet port of the membrane device and into downstream piping. This, of course, is disadvantageous to the user of the dry gas system. Furthermore, the feed gas during initial flow at low pressure, carries a larger percentage of water vapor than when the compressor has reached its normal operating pressure and is delivering the gas in compressed form after a large portion of the moisture has been removed as liquid in the compressor aftercooler. As a result, this larger percentage of excess water vapor is forced to condense as the pressure in the membrane device increases rapidly at a substantially constant temperature.
For some membrane dryers, e.g., the above-mentioned CACTUS.RTM. air dryer, the membrane material itself has a reasonably high adsorption capacity for water. Therefore, if action has been taken to purge the membrane with a dry gas source, the excess initial moisture will be adsorbed within the mass of the membrane preventing most of it from reaching the dry outlet port until the system pressure has been reached. Once the system pressure has been reached, the moisture exits the dryer with the permeate gases.
It is an object of this invention to provide an improved process for dehumidification of a compressed gas using a membrane cartridge dryer. The improvement in the process results in the ability to provide substantially dry gas immediately upon process startup while reaching normal operating pressure, without the requirement of purging or sweeping the membrane cartridge during the period of time in which the compressor is turned off.
The advantages provided by the present invention include an improved gas dehumidification process which does not require frequent replacement of any part of the apparatus. The process and apparatus require relatively small amounts of energy and the apparatus is economically fabricated and efficient in operation.